1. Field of the Invention
The present invention generally relates to a lens, and, more particularly, to a fixed-focus lens.
2. Description of Related Art
Referring to FIG. 1, a conventional fixed-focus lens 100 for a rear projection television (RPTV) is disclosed in U.S. Pat. No. 6,452,728. The fixed-focus lens 100 includes a first lens group 110, a second lens group 120 and a third lens group 130 arranged in sequence from an object side to an image side. A light valve 50 is disposed on the image side. The first lens group 110 includes four lenses 112, 114, 116 and 118 all having negative refractive powers. The lens 112 is an aspheric lens. The second lens group 120 includes two lenses 122 and 124. The lenses 122 and 124 have positive and negative refractive powers, respectively, and the lenses 122 and 124 constitute a cemented lens 121. The third lens group 130 includes six lenses 131, 132, 133, 135, 136 and 137. Refractive powers of these six lenses 131, 132, 133, 135, 136 and 137 in sequence from the object side to the image side are positive, negative, positive, positive, positive and positive. In addition, the lenses 132 and 133 constitute a cemented lens 134, and the lens 137 is an aspheric lens.
In the fixed-focus lens 100, because the third lens group 130 is close to the light valve 50 and the light valve 50 is close to a light source (not shown) generating heat, the third lens group 130 normally has a higher temperature relative to other lens groups. In general, the lenses of the third lens group 130 are made of glass to resist the high temperature. However, the last lens 137 of the third lens group 130 is an aspheric lens, and the cost of the fixed-focus lens 100 is increased when a glass aspheric lens, which is more expansive, is adopted as the lens 137. If a plastic aspheric lens, which is less expensive, is adopted as the lens 137 for reducing the cost, the lens 137 is easy to deform under the high temperature so that the reliability of the fixed-focus lens 100 is worse.